“To sum up: at various stages of embryonic development, and at various structural levels, we find different biochemical mechanisms, but analogue principles at work. At every stage and level the game is played according to fixed rules but with flexible strategies (although their flexibility is normally hidden from the eye and revealed only by the transplantation and grafting techniques of experimental embryology). The overall rules of the game are laid down in the complete set of instructions operative at any level at any time is triggered off by messages from the inter- and extra-cellular environment, which vary in character according to structural level and developmental stage: fertilizing agents, cytoplasmic feedbacks, direct-contact evocators, hormones, and other catalysts.”